Ion implantation and deposition are used to improve the surface characteristics of a variety of materials, including metals, ceramics, and plastics. Ion implantation and deposition allow new materials to be produced, having new surface properties, without the thermodynamic constraints of more conventional techniques. In particular, ion implantation and deposition can be used to improve greatly the friction, wear, and corrosion resistance properties of the surfaces of metals. The properties of ceramic components and ceramic cutting tools can also be improved by ion implantation or deposition.
In the conventional ion implantation process, ions are formed into a beam and accelerated to high energy before being directed into the surface of a solid target. The relatively high cost of this process has limited its use to high unit cost items having very special applications. A significant factor in the substantial production costs associated with conventional ion implantation techniques is that significant, and time-consuming, manipulation of the ion beam and the target is required to obtain implantation over the entire surface of a three dimensional target. In the conventional ion implantation method, the ions are extracted from a plasma source and focused into a beam, which is accelerated to the desired energy, and then rastered across one face of the target, to uniformly implant the surface of that face. Because of the line-of-sight nature of this ion implantation technique, a manipulator platform or stage is required which can support the target for rotation in the beam so that all sides of the target can be implanted. The need to manipulate a three dimensional target to allow all sides of the target to be implanted adds cost and complexity, constrains the maximum size of the target which can be implanted, and increases the total time required to obtain satisfactory implantation of all target surfaces for relatively large targets. Because the ions travel to the target in a largely unidirectional beam, it is often necessary to mask targets having convex surfaces so that ions are allowed to strike the target only at angles substantially normal to the target surface. Normal incidence of ions to the surface is preferred since, as the difference in the angle of the incidence from the normal increases, sputtering increases, and the retained net dose of implanted material in the target decreases. It would be impossible to use this method of ion implantation to implant, or deposit, materials on the inner surface of a cylindrical object.
Plasma source ion implantation (PSII) provides significantly improved production efficiencies in ion implantation of three dimensional materials by achieving implantation from all sides of the target simultaneously. This method was introduced in U.S. Pat. No. 4,764,394, entitled Method and Apparatus for Plasma Source Ion Implantation, issued to John R. Conrad, the disclosure of which is incorporated herein by reference. In the PSII process the target to be implanted is surrounded by the plasma source within an evacuated chamber. A high negative potential pulse is then applied to the target relative to the walls of the chamber to accelerate ions from the plasma, across the plasma sheath, toward the target, in directions substantially normal to the surface of the target at the points where the ions impinge upon the surface. Multiple pulses may be applied between the target and the chamber walls in rapid succession to perform multiple implantations until a desired concentration of implanted ions within the target object is achieved.
For PSII implantation the ion source plasma surrounding the target object is formed by introducing the ion source material, in a gas or vapor form, into the highly evacuated space within the confining chamber. The gaseous material may then be ionized in a conventional manner. Consequently, a plasma is formed which completely surrounds the target object itself so that ions may be implanted into the target from all sides, if desired. Since the target need not be manipulated, complicated target manipulation apparatus is not required. Multiple targets, properly spaced within the plasma, may be implanted simultaneously by the PSII process.
The PSII process can also be used to provide surface coatings through ion deposition. For ion deposition of thin films the voltage level applied to the target is reduced. Since the energy of the ions impacting the target surface is also reduced, the ions will not be driven deeply into the target surface but will tend to deposit on, or just under, the surface. At low energies, and with the appropriate plasma composition, a diamond-like carbon (DLC) coating can be produced from a methane or acetylene plasma. These DLC coatings are characterized by extremely high hardness, low friction and chemical inertness.
Pure metal, alloy, or metallic compound coatings have also been deposited using the PSII process in an ion-assisted deposition (IAD) mode. In the IAD process a radio frequency voltage source is applied to a sputter cathode made of the metal to be deposited via a capacitively or inductively tuneable matching network. This generates a plasma in a gas such as argon whose ions then impact on the cathode, sputtering material therefrom, which is then drawn by an electrical pulse applied to the target, for deposition on the surface of the target.
It is difficult in such plasma processing to provide a uniform surface coating on the inside of a cylinder, and uniform implantation or deposition on the outer surface of a cylinder is sometimes also difficult, especially in batch processing where many cylindrical targets are to be mounted in a vacuum, because of a lack of uniformity of the plasma surrounding, or within, the cylindrical target throughout the implantation/deposition period. One prior technique for obtaining uniformity on cylindrical surfaces involved rotating the target object. However, this is relatively difficult and expensive to do within the vacuum of a PSII chamber and defeats one of the advantages of PSII over the conventional ion beam technique.
In addition, typically during IAD and DLC deposition processes, the walls of the PSII vacuum chamber become contaminated by the sputtered, or otherwise deposited, material. This necessitates the frequent cleaning of the inside of the chamber, or the use of disposable stainless steel liners for the entire chamber.